rudiment

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A fundamental element, principle, or skill, as of a field of learning.
• noun Something in an incipient or undeveloped form.
• noun Biology An imperfectly or incompletely developed organ or part.

from The Century Dictionary.

• To furnish with first principles or rules; ground; settle in first principles.
• noun Anything which is in an undeveloped state; the principle which lies at the beginning or bottom of any development; an unformed or unfinished beginning.
• noun An element or first principle of any art or science; especially, in the plural, the beginning, first steps, or introduction to any branch of knowledge; the elements or elementary notions.
• noun In biology: That which is rudimentary; that which is in its first or an early stage of development, which may or may not be continued; the beginning or foundation of any part or organ: as, the rudiment of the embryo which is to go on to maturity; the rudiment of an organ whose further development has been arrested or aborted
• noun That which is vestigial; a vestigial or aborted part, organ, or structure; an abortion; a vestige.

from the GNU version of the Collaborative International Dictionary of English.

• transitive verb To furnish with first principles or rules; to insrtuct in the rudiments.
• noun That which is unformed or undeveloped; the principle which lies at the bottom of any development; an unfinished beginning.
• noun Hence, an element or first principle of any art or science; a beginning of any knowledge; a first step.
• noun (Biol.) An imperfect organ or part, or one which is never developed.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A fundamental principle or skill, especially in a field of learning (often in the plural).
• noun Something in an undeveloped form (often in the plural)
• noun biology A body part that no longer has a function
• noun music In percussion, one of a selection of basic drum patterns learned as an exercise.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the remains of a body part that was functional at an earlier stage of life
• noun the elementary stages of any subject (usually plural)